This invention embodies a new and novel process for the preparation of encainide (I) ##STR1## Encainide, chemically, 4-methoxy- 2'-[2-(1-methyl-2-piperidyl) ethyl ]benzanilide, is a member of a series of antiarrhythmic 2-phenethylpiperidines bearing amide substituents in the ortho-position of the phenyl ring. Encainide hydrochloride is also referred to in the literature as MJ 9067-1 (USAN and the USP Dictionary of Drug Names, 1980, p. 122, United States Pharmacopeal Convention, Inc., 12601 Twinbrook Parkway, Rockville, MD. 20852, Library of Congress Catalog Card No. 72-88571). Currently, encainide is undergoing clinical evaluation as an effective antiarrhythmic agent.
Previous syntheses of encainide and closely related compounds are described in the following references.
Dykstra, S. J., et al., J. Med. Chem., 16, 1015-1020, (1973).
S. J. Dykstra and J. L. Minielli, U.S. Pat. No. 3,931,195 patented Jan. 6, 1967; U.S. Pat. No. 4,000,143 patented Dec. 28, 1978; U.S. Pat. No. 4,064,254 patented Dec. 20, 1977.
Byrne, J. E., et al., J. Pharmacology and Experimental Therapeutics, 200, 147-154 (1977).
Another process, different from syntheses disclosed in the above-cited references, is described in U.S. Pat. No. 4,394,507, which issued July 19, 1983.
The instant process differs from these previously disclosed processes and offers advantages that will be evident from the description of the invention.
The following references relate to condensation of aromatic aldehydes with quaternized .alpha.-picolinium halides.
a. Horwitz, Journal of Organic Chemistry, 21, 1039-1041 (1956). PA1 b. Stanek, et al., Chem. Abstracts, 49, 314h (1955). PA1 c. Phillips, Journal of Organic Chemistry, 12, 333-341 (1947).
The condensation process disclosed and discussed in the above references a-c does not proceed readily with o-nitrobenzaldehyde, affording yields of only 40% or less of condensation product.
There is nothing in any of the above references or any other prior art that would suggest or make obvious the novel process embodied in the present invention. Summary of the Invention
This invention relates to a new process for the preparation of encainide which has the formula ##STR2## or the hydrochloride acid addition salt thereof starting with .alpha.-picoline, o-nitrobenzaldehyde, and anisoyl chloride. Detailed Description of the Invention
The following flow chart, Scheme 1, illustrates the preparation of encainide from readily available starting materials utilizing the instant process. ##STR3##
This invention relates to a new and novel synthesis of encainide. It was an object of the present invention to develop a method of synthesis for encainide having a minimal number of steps and which would start with o-nitrobenzaldehyde which is now available commercially as a bulk chemical commodity. Another object of the invention was to design a process which was more economical in terms of consumption of expensive noble metal catalysts, e.g. prior art processes require two hydrogenation steps and/or use of both platinum and palladium. Finally, it was an object of the present invention to produce a final product in the form of a pure white solid, lacking the varying degrees of pink color which characterized product produced by prior art methods.
The objectives of the present invention have been achieved by the process which will be described in detail below.
The first step of the process outlined in Scheme 1 involves the condensation of o-nitrobenzaldehyde with the picolinium methyl sulfate salt (V; generated in situ from dimethylsulfate and .alpha.-picoline). The quaternizing use of dimethylsulfate instead of the commonly employed methyl halides facilitates the aldehyde +.alpha.-picoline condensation and leads to very high yields of the alcohol product IV. Simple mixing of the reactants in an inert organic solvent, such as methylene chloride, results in high yield (92-94) of the alcohol product IV which can be conveniently isolated by filtration. This facile, high yield condensation, achieved in two hours at room temperature as opposed to prior art conditions of 50 hours in refluxing acetic anhydride, when .alpha.-picoline and o- nilrobenzaldehyde are condensed appears to be due to the use of dimethylsulfate as the quaternizing agent.
In order to capitalize on this significant improvement in product yield from the condensation reaction, it was necessary to overcome poor results in the dehydration step of IV to III. Attempts to dehydrate IV to III (step 2) in a manner analogous to that of the quaternary halide salts, specifically the bromide, met with significant complications and low yield. It was subsequently discovered that the addition of a catalytic amount of potassium acetate to the dehydrating mixture of acetic acid and acetic anhydride resulted in production of the olefin III in greater than 90% yield. This olefin product III, as envisioned, is now suitably disposed for the simultaneous reduction of all three functional moieties (nitro, olefin, pyridinium) to form the requisite functional moieties (amino, alkylene, piperidine) of the encainide molecule. These reductions are simultaneously accomplished in step 3 by catalytic hydrogenation using platinum metal catalyst to give the diamine intermediate II. The hydrogenation process is accomplished using an alcoholic solution of III, 95% ethanol being preferred. The diamine product II is isolated as an oil following neutralization and extraction with an immiscible solvent such as methylene chloride. As an added advantage, the Pt catalyst recovered from this step does not suffer significant poisoning, as in prior art processes, but may be recycled.
The final reaction (step 4) leads to the desired final product, encainide hydrochloride, in form of a white solid lacking the contaminating pink coloration often appearing in product produced by other processes. It was discovered that this conversion of II to encainide hydrochloride of high purity could be accomplished by simply adding anisoyl chloride to a solution of the diamine intermediate II in an appropriate organic solvent such as acetone, acetonitrile, or an alcohol. An appropriate solvent is one in which compound II and p-anisoyl chloride are soluble but encainide HCl is not. The most preferred solvent for this step is acetone. In the use of alcohols, C.sub.2-5 alkanols such as propanol, butanol, and so forth are intended. This facile acylation reaction also proceeds very cleanly with a surprising lack of any significant amounts of hydrochloride salt products of II appearing. Liberated HCl is effectively scavenged by the base form of newly formed 1. If required, encainide free base of high purity may be obtained in the usual manner from the encainide hydrochloride produced in the subject process. Recrystallized encainide hydrochloride is obtained in 55-60% yield from compound III in the new process.
In summary, the process of the instant invention meets the objectives set forth hereinabove. It is a new four-step process which uses o-nitrobenzaldehyde as a starting material, achieves economic savings with the single catalyst hydrogenation step employed, and provides pure product lacking the color impurities which frequently occurred in prior art processes.